DRY-MATTER AND LEAF-AREA PARTITIONING, BUD FERTILITY AND SECOND SEASON GROWTH OF VITIS-VINIFERA L - RESPONSES TO NITROGEN SUPPLY AND LIMITING IRRADIANCE

Potted Vitis vinifera L. plants were grown under controlled environmen tal conditions at five different levels of nitrogen (0, 1, 5, 10, 100 mM NH4NO3) in combination with two different levels of irradiance (30 and 140 mu mol m(-2) s(-1) PAR, respectively) during bloom. The immedi ate, whole season and second year effects on vegetative growth were as sessed, and bud fertility and rate of development were evaluated. The optimum N addition level was 1 mM NH4NO3 for root growth and 5 mM for shoot growth, respectively, both after bloom and at the end of the fir st growing season. This growth response to N supply became apparent on ly in the higher light treatment and was mainly due to an N-induced en hancement of leaf and lateral shoot growth. Low-light stress also stro ngly enhanced the number of new leaves and laterals, but total dry mat ter production was reduced and did not respond to N nutrition. Light r estriction increased the specific leaf area by 52 % and the leaf area ratio by 37 % but did not affect the leaf weight ratio. The leaves of N-deficient vines, in particular in combination with light stress, sen esced earlier than those of vines with sufficient or excessive N avail ability. The light effect on shoot growth in the second season was inv erted compared with the first season, and the peak response to N suppl y was shifted towards 100 mM NH4NO3. Limiting light conditions during inflorescence initiation severely reduced the bud fertility but advanc ed the date of bud break and enhanced the rate of development of the n ew shoots in the subsequent season. The optimum N supply rate for both bud fertility and development was 5 mM NH4NO3. Bud mortality was not affected by either treatment factor. These data indicate that the bloo m period is critical for reproductive development of grapevines, with high sensitivity to environmental stress. They also emphasize the impo rtance of nutrient reserves in the permanent structure for both compen satory and early season growth.